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How does a NPN transistor work?

how does a npn transistor work

This article is aimed at beginners and so is simplified to the extreme. There are complete books written on “how does a NPN transistor work”, with pages and pages of mathematical formulas. I’m not going to delve into anything like that here.

A transistor is an electronic component that is operated by an electrical current. It is commonly configured to work as an amplifier or a switch.

In order to understand “how does a NPN transistor work”, you first need to understand what a NPN transistor is. A NPN transistor is a type of bipolar junction transistor (BJT). It has three connections that are called:-

1. The base.

2. The collector.

3. The emitter.

The symbol for a NPN transistor is shown below.

how does a npn transistor work npn transistor symbol

In order to explain how a NPN transistor works, it’s easier to show it in a circuit.

how does a npn transistor work switched off

In the circuit above the transistor is off and the LED is off because no current is flowing through it.

To turn it on, you need to apply a voltage at the base of over .7V then the transistor will turn on.

This can be done by connecting a resistor to the base of the transistor and to the positive of the power supply as in the diagram below.

how does a npn transistor work switched on

A small current flowing to the base causes a larger current to flow through the collector to the emitter.

That is the simplest way to show a transistor in operation. Effectively the transistor is operating as a switch.

A transistor doesn’t just have to be on or off. The more current that flows in the base the more current flows through the collector and emitter, so it effectively amplifies. The amount that it amplifies is known as the gain.

What’s so fantastic about that you may think? On its own not a lot but it’s when you combine that function with other stuff it starts to become more interesting. You could connect up a light sensitive resistor to the input stage and have the transistor turn a light on when it gets dark. Once you start thinking there are endless possibilities.

Transistors are at the heart of almost everything electronic. Even if they are not used directly they will still be there. The 555 timer integrated circuit contains 25 transistors all etched onto a small piece of silicon but still functioning as transistors.

If you want to know how a PNP transistor works read here.